DNA normally exists in double-stranded form, each strand composed of
molecular subunits called bases. The strength of attraction between the two strands
depends on proper matching of their bases. The better the matching, the stronger the
attraction, and the higher the temperature required to melt them (separate the strands).
DNA distance is a measure of the extent of base matching of DNA strands from different
species, and can be estimated from the effect of base mismatching on the melting
temperature. One unit of DNA distance is equivalent to a melting point depression of 1șC.
DNA distances have been used to estimate the degree of genetic
similarity between species. "Family trees" have been constructed from DNA
distances for most families of songbirds. Species classified as closely related usually
cluster together in a distinct group with DNA distances of less than about 5 to 7. Species
groups often are separated from other such groups by distances of 10 or more units. At
distances more than about 8 to 10, branches tend to be so close together that the
relationships among groups may be difficult to interpret. In addition, the branching
pattern at greater DNA distances often conflicts with branching patterns derived from
other methods of classification, leaving one to wonder which system is best.
The clustering of species into distinct groups suggests they may be
related by common ancestry. DNA distances seem to be useful in grouping species into
higher taxonomic categories. The method sometimes suggests hypotheses of relationship
between groups of species, or for unique species, that may not have been seriously
considered previously. This is interesting, but, as with other methods of systematics,
difficulties remain. One important question is the limit of resolution of the method. It
seems likely that the reliability of the method decreases as DNA distance increases, and
the method is probably best used for grouping species rather than determining
relationships among such groups. Nevertheless, DNA distance, along with other methods of
comparing species, will continue to be of interest to all who are interested in the
relationships among living organisms.

Fossil reefs are reported from many parts of the world, especially
for the Paleozoic era. While binding of reef components by carbonate-secreting organisms
is an undisputed fact for modern reefs, such binding for Paleozoic reefs is not generally
observed. Other major differences include the size, taxonomy and abundance of
reef-building organisms and the composition and coarseness of matrix material. Such
differences make use of the term "reef" for the Paleozoic structures highly
controversial.